1. Field of the Invention
The present invention relates to balanced SAW filters having a balanced-to-unbalanced conversion function, and more particularly, to a balanced SAW filter including a 5-IDT longitudinally coupled resonator-type surface acoustic wave filter including five IDTs.
2. Description of the Related Art
As band-pass filters, surface acoustic wave filters have been widely used for RF stages of mobile communication apparatuses, such as cellular phones. In recent years, there has been a demand for surface acoustic wave filters of this type having a balanced-to-unbalanced conversion function, that is, a so-called balun function. Under such circumstances, recently, longitudinally coupled resonator-type surface acoustic wave filters have been used as band-pass filters for RF stages since longitudinally coupled resonator-type surface acoustic wave filters support higher frequencies and easily achieve the balanced-to-unbalanced conversion function.
A longitudinally coupled resonator-type surface acoustic wave filter having a balanced-to-unbalanced conversion function is disclosed in Japanese Unexamined Patent Application Publication No. 2004-96244 described below. FIG. 13 is a schematic plan view showing an electrode structure of the surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 2004-96244. A surface acoustic wave filter 101 includes five IDTs 102 to 106 disposed along a surface-acoustic-wave propagation direction. Reflectors 107 and 108 are disposed on both sides in the surface-acoustic-wave propagation direction of an area in which the IDTs 102 to 106 are disposed.
The IDTs 103 and 105 are commonly connected to an unbalanced terminal 111 with a one-port surface acoustic wave resonator 109 therebetween. The IDT 104, which is disposed in the middle, is divided into two portions in the surface-acoustic-wave propagation direction and includes first and second sub-IDT portions 104a and 104b. The IDT 102 and the sub-IDT portion 104a are commonly connected to a first balanced terminal 112. The sub-IDT portion 104b and the IDT 106 are commonly connected to a second balanced terminal 113.
With the five IDTs 102 to 106 connected as described above, the balanced-to-unbalanced conversion function is achieved. The 5-IDT longitudinally coupled resonator-type surface acoustic wave filter 101 achieves the advantages of reducing the insertion loss within a pass band, allowing easy adjustment of terminal impedance, and achieving further reduction in size. In addition, the total number of electrode fingers of the IDTs 103 and 105 connected to the unbalanced terminal 111 is smaller than the total number of electrode fingers connected to the balanced terminal 112, that is, the total number of electrode fingers of the IDT 102 and the sub-IDT portion 104a, or the total number of electrode fingers connected to the balanced terminal 113, that is, the total number of electrode fingers of the IDT 106 and the sub-IDT portion 104b. Thus, an impedance of the balanced terminals 112 and 113 is two to four times the impedance of the unbalanced terminal 111.
However, depending on a design parameter, the 5-IDT surface acoustic wave filter 101 generates an unwanted ripple within a pass band.